Conventional polycrystalline diamond drill or drag bits used for drilling subterranean formations generally have a plurality of polycrystalline diamond cutting elements that protrude outwardly from the bit surface and that are arranged in blades that each extend along an axis running along the bit from a face portion of the bit, over a shoulder portion, and to a gauge portion of the bit. In service, the cutting elements disposed at the shoulder portion are typically exposed to more aggressive wear due to both axial and radial forces that are directed onto the bit. Additional blades are oftentimes placed along the shoulder portion, increasing the density of cutting elements along the shoulder portion and, thereby, minimizing the effect of such aggressive wear.
As the drill bit is rotated in the bore hole the engagement of each cutting element within the hole creates forces that are imparted to the drill bit. The sum of these forces result in the formation of a unified force of single direction that is imposed on the drill bit and that causes the drill bit to track to one side of the hole in a direction away from its axis of rotation. As the drill bit is rotated within the hole out of its axis of rotation, interaction of the cutting elements against the hole side wall causes the drill bit to vibrate. The vibrations cause abnormally aggressive wear and impact damage to the drill bit, ultimately reducing bit service life.
Bits known in the art have been configured to reduce whirl and are referred to as Anti-Whirl bits. Such a bit is disclosed in U.S. Pat. No. 5,010,789 and is typically configured having low friction pads disposed along the shoulder and gauge portion of the bit. Accordingly, to accommodate the low friction pads, the shoulder and gauge portions are constructed having a reduced cutter element density, e.g., up to 20 percent fewer cutter elements than a conventional polycrystalline diamond drill bit. To ensure effective functioning of the low friction pads in reducing whirling, it is necessary that the operating parameters of the bit, e.g., revolution speed (RPM) and weight-on-bit, be limited to a defined window so that interaction between the cutter elements and the hole are sure to impose a force on the bit that is directed along the low friction pads. Operating the Anti-Whirling bit outside of the defined window can cause a force to be directed to the shoulder and gauge portion of the bit having the cutting elements. This not only causes the bit to move off-track from its axis of rotation, or whirl, but also causes aggressive wear to take place at the aforementioned shoulder and gauge portion of the bit. Accordingly, operating the anti-whirl bit outside of the limited window of operating parameters can ultimately result in the premature failure of the bit. Because the Anti-Whirl bit can only operate within the narrow window, its use is limited to only particular applications.
Core or coring bits are known in the art and are configured to form a core portion from the formation being drilled by the rotational action of the bit. The core bit is configured having a cutting portion disposed along a face and shoulder portion of the bit. The cutting portion extends a distance into an annular opening in the center of the face portion of the bit. The cutting portion may comprise a plurality of cutting elements that project outwardly away from the bit face and shoulder surfaces. Operation of the core bit causes the cutting portion of the bit to engage the formation, creating a core portion that passes into and through the annular opening.
The formation of the core portion and housing of such portion with the annular opening during drilling action of the bit does have some effect on centering the bit. However, whatever effect the core bit may have on centering the bit is overcome by the forces that are directed on the bit during drilling operation by interaction of the cutting portion of the bit with the bore hole, causing the coring bit to whirl and rotate off track from its rotational axis. Additionally, in conventional core bits the bit must be removed from the hole after drilling a short length so that the core can be removed. Therefore, the use of such core bits are typically limited to drilling short sections of a bore hole. Core bits are also only used in drilling straight line holes and, thus, cannot be used for directional drilling.
Some core bits are configured having a core ejector mechanism that facilitates the removal of the core portion from the bit without having to remove the bit from the bore hole. U.S. Pat. Nos. 4,694,916 and 3,323,604 each disclose a coring drill bit that includes a type of core breaker that facilitates breakage of the core and transportation of the broken core portion through the bit to the annulus. The advantage of a core comprising such an ejector is that it permits use of the bit in drilling long sections, as the core formed by the bit is ejected from the bit towards the annulus during the drilling operation. However, use of the bit comprising a core ejector is limited to drilling hard formations, to facilitate removal of the core portion, and to straight-line drilling.
U.S. Pat. No. 3,635,296 discloses a drill bit constructed having a crown portion comprising a matrix of cutting elements disposed thereon, and an annular cavity disposed within a center of the bit about the axis of rotation. The cutting elements extend a distance into the annular cavity. A cutting wheel is located within the annular opening and has a cutting surface positioned perpendicular to the opening. During drilling operation of the bit the matrix of cutting elements act to form a core portion from the formation that travels into the annular cavity. The cutting wheel acts to crush the core portion once it has traveled a distance through the cavity. The broken core portions are removed from the cavity through passages via hydraulic transport provided by drilling fluid.
Although this bit embodiment does have some effect on aiding the centering of the bit along its axis of rotation during operation, e.g., by the action of forming a core portion and housing the same within the cavity, the bit does not include a means for directing the forces imposed on the bit to a designated portion of the bit. Accordingly, the undirected forces imposed on the bit by the interaction of the cutting elements with the bore hole go unchecked and result in bit whirl and aggressive wear of the bit's cutting elements, thereby, reducing the service life of the bit.
None of the above-mentioned bits known in the art are configured in a manner that is effective in reducing bit whirling and keeping a drill bit on-line with its rotational axis during drilling. Furthermore, the operation of these bits are limited to narrow operating parameters, such as for short-length drilling, specific drilling parameters, and for straight-line only drilling.
It is therefore desirable that a polycrystalline diamond drill bit be constructed in a manner that will reduce or eliminate bit whirling during operation in drilling subterranean formations under a variety of operating parameters. It is desirable that the drill bit be constructed to accommodate straight-line or directional drilling, for either long or short length hole sections. It is desirable that the drill bit be configured in a manner that prevents bit whirling without sacrificing the service life and efficiency of the bit when compared to conventional drill bits. It is further desirable that the drill bit be constructed in a cost effective manner using conventional manufacturing techniques and using conventional materials of construction.